Mozilla recently placed Thunderbird in a maintenance-only state, where active feature development has ceased but stability, security, and speed improvements will continue to be merged. This is why Thunderbird is on v17 instead of Firefox's v21 after graduating versions in lockstep with Fx for a while.
It doesn't mean they won't accept features, they've just stated that they're not going to be focusing any of their development wherewithal on these.
Personally I think it's a good move as TB has plenty of features for a mail client but could really do with some optimization to speed and memory usage.
I was just recently trying eM Client, was happy at the beginning as it has nice interface and integrates with GMail contacts and calendar. Was considering even buying the license. But then I started having contact local/remote sync conflicts without having them changed on either side. And then while the modal was blocking UI, it apparently was blocking the scheduler, so it was queuing all mailbox sync operations and performs ALL of them multiple times after modal is closed...
So I guess back to Opera Mail. Other ones like TB, Sylpheed don't cut it for me.. I guess they have UI and network actions within the same thread, and that makes them get stuck.
The sync conflicts are certainly not a usual thing to happen if no change has happened on either side.
Also the modal UI blocks only the one sync queue - i.e. the one for contacts & calendars in that account. All other accounts including mails for that same account should continue syncing.
I'd love to help you with those issues - you can contact me at grafnetr@emclient.com
There was a thread on HN a few months back that talks about its unique security/password sharing model. It's clear that the developers put a whole lot of love into it.
It doesn't mean they won't accept features, they've just stated that they're not going to be focusing any of their development wherewithal on these.
Personally I think it's a good move as TB has plenty of features for a mail client but could really do with some optimization to speed and memory usage.